In their early development neonatal and fetal animals acquire passive immunity as a consequence of the selective transport of maternal immunoglobulin G (IgG) from maternal serum to the circulation of the young. Other immunoglobulins do not traverse the tissue forming the maternofetal interface; the process is specific for IgG. The yolk sac in fowl and rabbits, the gut in many rodents, and the placenta in most other mammals are the primary selective barriers between mother and young. In the chick yolk sac, the epithelial cells fronting on the yolk appear to be responsible for the selective transport of IgG. It is postulated that IgG binds to specific receptors localized to the plasma membrane of the apical surface of the epithelial cells. Concommitant with or subsequent to binding, IgG becomes localized to specialized areas of the plasma membrane distinguished by a bristle-like coating on the cytoplasmic face. These coated pit regions subsequently invaginate to form coated vesicles containing IgG bound to the interior face of the membrane. Coated pits and coated vesicles appear in all eucaryotic cell types and are thought to function in protein transport and membrane recycling. Maternal-fetal transport of IgG across the chick yolk sac is one of the few systems where it can be documented that coated pits and coated vesicles are involved in the transport of a specific protein. The objectives of the proposed study are to define the initial events in the process of maternofetal IgG transport across the chick yolk sac and to examine the relationship of these events to the proposed role of coated pits and coated vesicles in specific protein transport. Specific binding of IgG to a membrane localized receptor appears to be the prerequisite event for transport of intact IgG to the fetal chick circulation. Thus, the biochemical parameters of specific IgG binding to intact yolk sac fragments and to membrane preparations derived there from will be defined using radiolabelled IgG. Once the optimal conditions for and characteristics of specific IgG binding are defined, the morphological characteristics and localization of binding will be studied at the level of resolution afforded by the electron microscope. The localization of IgG labelled with an electron dense marker will be studied under the conditions defined as optimal for specific IgG binding. This coordinated biochemical and cytological app (Text Truncated - Exceeds Capacity)